1. Field of the Invention
The present invention relates generally to satellite communications and, more particularly, to dynamic power control with an adaptive reference level in a point-to-point satellite communications system.
2. Description of Related Art
Communication satellites have the capability of receiving uplink signals from earth-based terminals, amplifying and translating the uplink signals into downlink signals, and retransmitting the downlink signals to receiving terminals. Such communications systems permit two types of communications. In a typical broadcast system, such as for distributing television programming, an uplink signal from a single broadcast transmitter is retransmitted by the satellite for simultaneous reception by multiple receiving terminals. In two-way systems, such as for normal telecommunications, uplink signals from many uplinking terminals are transmitted with no central coordination to a single satellite. Each uplink signal is addressed to and retransmitted for reception by one or more receiving terminals. Both types of systems permit concurrent receipt and processing of multiple uplink signals.
The satellites in both broadcast and two-way systems are capable of receiving uplink signals from multiple terminals using time-division multiple access (TDMA), frequency-division multiple access (FDMA) or a combination of the two access methods. In TDMA, each terminal uses the entire uplink bandwidth for a portion of the time. A synchronization arrangement which controls the time of transmission of each terminal is required. In an ideal situation, each terminal transmits uplink signals that arrive at the allotted time without overlap or gaps. Unfortunately, in the real world terminals sometimes transmit TDMA uplink signals that overstep their allotted time slot, thereby jamming the signals of other terminals. In these cases, the relative transmission timing for the offending terminals must be controlled to ensure the uplink signals arrive in the proper time slot.
In FDMA, the uplink bandwidth is subdivided and portions are assigned to different terminals. Power levels of the uplink signals must be controlled to reduce interference between the subdivisions. An FDMA uplink signal will leak, due to hardware imperfections, into the adjoining frequency bandwidth subdivision. If the uplink signal has too much power, the FDMA leakage will jam the adjoining subdivision. In this case, the relative signal powers of the uplink signals in adjoining subdivisions must be adjusted to minimize interference due to signal leakage.
In a broadcast satellite communications system, transmission stations transmit to a satellite uplink signals which are broadcast and simultaneously received by multiple receiver stations. The transmission station transmits an uplink signal at a predetermined uplink frequency to an orbiting satellite. The satellite receives the uplink signal and upconverts or downconverts the uplink signal to a downlink signal at a predetermined downlink frequency. The downlink signal is then retransmitted in a broadcast beam for simultaneous reception by multiple receiver stations. Typically, the transmission station is within the area encompassed by the broadcast beam and, therefore, is capable of receiving the downlink signal.
In broadcast systems as described, uplink power control and time synchronization can be performed at the transmission station. Power control is typically performed by the transmission station measuring the power of a beacon or a communications carrier signal in the broadcast beam from the satellite. The beacon or signal is transmitted with a predetermined power in either the uplink frequency band or the downlink frequency band. The transmission station measures the power of the received beacon or signal, and determines whether the uplink signal power should be adjusted to allow for variations in signal fade and interference at the uplink signal frequency. In these systems, the uplink signal powers can be increased or decreased as is necessary to overcome signal fade because the uplink signals from one transmission station do not interfere with the uplink signals of the other transmission stations of the system.
Time synchronization in broadcast satellite communications systems and systems having central coordination of uplink signal transmissions is usually performed by the transmission station measuring its own transmission as it appears in the downlink signal. The timing relationship between the uplink signal and the downlink signal is constant as the uplink signal is received, converted to the downlink signal, amplified and retransmitted by the satellite. Therefore, the time of receipt of the downlink signal at the transmission station can be used to adjust the transmission time of the uplink signal to ensure that the uplink signal arrives at the satellite at the allotted time.
Previous alternatives for uplink power control and timing synchronization are not applicable in the uncoordinated two-way systems as described herein. Specifically, the two-way system described herein interconnects multiple, geographically disparate spot beam coverage areas with a regenerative satellite payload and no centralized control station. The payload demodulates uplink signals into their constituent packetized bit streams and routes the packets to the downlink spotbeam(s) specified within the packet header. Thus, the originating terminal may or may not receive the downlink manifestation of the uplink signal. Moreover, both the timing and the signal-to-noise ratio of the uplink signal are removed by the demodulation and routing operations. Demodulation reduces the signal to the binary information stream, and routing introduces random queuing delays. Thus, neither the timing nor the received signal strength is discernible from the downlinked data.
Another problem exists in systems where low power uplink terminals are used to reduce size, cost and power consumption. In these systems, the uplink terminals cannot indiscriminately increase their uplink signal power to compensate for other uplink signals that leak into their subdivisions. Even in systems with uplink terminals capable of transmitting signals with higher uplink signal powers, the terminals cannot indescriminantly increase their uplink signal power lest system runaway occur. Consequently, a power control strategy is necessary in systems without central coordinated uplinks and using either low power or high power uplink terminals wherein the terminals are self-policing and each control their own uplink signal power.
For these reasons, a need exists for a method for satellite-based uplink power control and time synchronization in satellite communications systems.